Release liners are used extensively in industry for a variety of purposes, including providing mechanical strength and support to manufactured articles, protecting manufactured articles during transportation and storage, and providing release properties to the manufactured article (when it is desired to withdraw the supported manufactured article from the release liners). For example, release liners are used in the transportation and storage of self-sticking floor tile to protect the adhesive surface that is present on these products until the point of use, at which time the release liner is easily removed and discarded.
Release liners may also be used as industrial tooling to manufacture articles from curable compositions. For example, a curable composition can be coated onto a release liner and cured, and then the resultant cured product can be subsequently removed for further processing, use, and/or distribution. If the release liner has a surface texture, the texture can be imparted to the cured article. Such products can be formed from compositions that may be cured using chemical crosslinking techniques, radiation cross-linking techniques, and the like. Radiation curable compositions are particularly useful in that such compositions can be coated and thereafter quickly cured, resulting in fast cycle times.
Various materials have been used to manufacture release liners. For example, release liners comprising polypropylene, polyethylene, polyesters, silicone rubbers, and various copolymers of these materials are well known in the art. Release liners of fluorinated polymers such as polytetrafluoroethylene are also known. However, many of these materials, such as polyethylene, polypropylene, and polyester, have relatively low heat distortion temperatures or lose their release properties at elevated temperatures. Consequently, these materials are limited to low temperature applications, e.g., temperatures below about 85.degree. C.
Furthermore, many of these materials require the use of a release agent to be generally incorporated into, or coated onto, the release liner in order for the liner to have the desired release properties. The use of a release agent, however, complicates the manufacturing process and can lead to the introduction of impurities into the finished product, sometimes with an accompanying reduction in desirable physical properties.
Additionally, many commercially available release liners are not amenable to use in processes in which materials supported upon the release liner are to be cured using radiation curing techniques, as through exposure to ultraviolet or electron beam radiation sources. Thus, some polymers used in conventional release liners lose their release characteristics or undergo physical distortions when irradiated with ultraviolet or electron beam radiation. For example, when release liners comprising silicone rubber are exposed to e-beam radiation, the e-beam radiation induces grafting and other chemical reactions in the release liner that causes the liner to bind to an article supported on its surface.
Other release liners absorb so much of the incident radiation that it is not feasible to cure materials supported upon the liner by irradiation through the release liner. Such through-curing is desirable in applications such as adhesive synthesis, adhesive cross-linking, radiation cure replication, or in situations where the material to be cured is sandwiched between two release liners.
Many prior art release liners are currently manufactured from fluorinated polymers such as polytetrafluoroethylene (commercially available under the tradename "Teflon" from E.I. duPont de Nemours and Company). While these release liners exhibit good release properties toward a variety of materials, they are too expensive to be economically feasible in many applications, as where the release liner will be disposed after a single use.
Many of the prior art release liners are also incompatible with the materials and compositions of interest in adhesive and microreplication applications. Specifically, many release liners exhibit large surface energy differences with low viscosity, molten, polymeric admixtures, causing them to suffer from problems such as dewetting during coating operations. In such cases, the molten admixture may tend to "bead up" on the surface of the release liner, instead of forming a uniform coating as desired. On the other hand, many materials on which a uniform coating may be readily formed do not provide the desired release properties.
There is thus a need in the art for a release liner which has low surface energy properties, is resistant to heat distortion at high temperatures, is compatible with radiation curing techniques, allows through-curing of radiation-curable compositions, and is relatively inexpensive. There is also a need in the art for a release liner which provides good release properties without exhibiting dewetting problems with respect to a molten admixture (e.g., a release liner which exhibits desirable release properties with respect to the finished article, but which exhibits good wet-out with respect to the molten admixture).
Another problem with many prior art release liners is their inability to provide good release properties to Interpenetrating Polymer Networks (IPNs). IPNs are networks of two or more polymers that are formed by independent polymerization of two or more monomers in the presence of each other so that the resulting independent crosslinked polymer networks are physically intertwined but are essentially free of chemical bonds between them (that is, there is produced an entangled combination of two crosslinked polymers that are not chemically bonded to each other). Some of the more important IPNs include simultaneous IPNs, sequential IPNs, gradient IPNs, latex IPNs, thermoplastic IPNs, and semi-IPNs. These and other types of IPNs, their physical properties (e.g., phase diagrams), and methods for their preparation and characterization, are described, for example, in L. H. Sperling and V. Mishra, "Current Status of Interpenetrating Polymer Networks", Polymers for Advanced Technologies, Vol. 7, No. 4, 197-208 (April 1996), L. H. Sperling, "Interpenetrating Polymer Networks: An Overview", Interpenetrating Polymer Networks, edited by D. Klempner, L. H. Sperling, and L. A. Utracki, Advances in Chemistry Series #239, 3-38, (1994), "Encyclopedia of Polymer Science and Engineering", p. 279, Vol. 8 (John Wiley & Sons, New York, 1984), and in L. ff. Sperling, "Interpenetrating Polymer Networks and Related Materials, "(Plenim Press, New York, 1981).
Due to their unique molecular structures, IPNs possess a number of very desirable physical properties. However, most prior art release liners exhibit very poor release properties with respect to IPNs, particularly some of the more desirable IPNs such as urethane acrylate IPNs. As a result, it is often difficult to make articles having a structured (e.g., microreplicated) surface from IPNs, nor is there a convenient method for making articles from IPNs that can be releasably coupled to a release liner. There is thus a need in the art for a release liner that provides good release properties for IPNs such as urethane acrylate IPNs, and that can be used to impart a structured or patterned surface to such IPNs.
These and other needs are met by the present invention, as hereinafter described.